diff --git a/mercurial/bundle2.py b/mercurial/bundle2.py
--- a/mercurial/bundle2.py
+++ b/mercurial/bundle2.py
@@ -299,7 +299,7 @@ def processbundle(repo, unbundler, trans
                 if key != parttype: # mandatory parts
                     # todo:
                     # - use a more precise exception
-                    raise error.BundleValueError(key)
+                    raise error.BundleValueError(parttype=key)
                 op.ui.debug('ignoring unknown advisory part %r\n' % key)
                 # consuming the part
                 part.read()
@@ -831,7 +831,8 @@ def handlereplycaps(op, inpart):
 @parthandler('b2x:error:unsupportedcontent')
 def handlereplycaps(op, inpart):
     """Used to transmit unknown content error over the wire"""
-    raise error.BundleValueError(inpart.params['parttype'])
+    parttype = inpart.params['parttype']
+    raise error.BundleValueError(parttype=parttype)
 
 @parthandler('b2x:error:pushraced')
 def handlereplycaps(op, inpart):
diff --git a/mercurial/error.py b/mercurial/error.py
--- a/mercurial/error.py
+++ b/mercurial/error.py
@@ -103,7 +103,10 @@ class BundleValueError(ValueError):
     """error raised when bundle2 cannot be processed
 
     Current main usecase is unsupported part types."""
-    pass
+
+    def __init__(self, parttype):
+        self.parttype = parttype
+        super(BundleValueError, self).__init__(parttype)
 
 class ReadOnlyPartError(RuntimeError):
     """error raised when code tries to alter a part being generated"""
diff --git a/mercurial/wireproto.py b/mercurial/wireproto.py
--- a/mercurial/wireproto.py
+++ b/mercurial/wireproto.py
@@ -806,7 +806,7 @@ def unbundle(repo, proto, heads):
     except error.BundleValueError, exc:
             bundler = bundle2.bundle20(repo.ui)
             errpart = bundler.newpart('B2X:ERROR:UNSUPPORTEDCONTENT')
-            errpart.addparam('parttype', str(exc))
+            errpart.addparam('parttype', exc.parttype)
             return streamres(bundler.getchunks())
     except util.Abort, inst:
         # The old code we moved used sys.stderr directly.